CROSS-TROPOPAUSE MASS-EXCHANGE AND POTENTIAL VORTICITY BUDGET IN A SIMULATED TROPOPAUSE FOLDING

To study stratosphere - troposphere exchange, an approach based on the nonconservation of potential vorticity (PV) is developed; this approa ch arises naturally if one defines the tropopause in terms of PV. The evolution of a tropopause fold simulated by a mesoscale model is studi ed, as well as the evolution of PV at the tropopause level. The PV fra mework also permits the identification of the physical processes respo nsible for the cross-tropopause exchange as either diffusive or diabat ic. In this model simulation, the diabatic processes are found to be t he most important in the exchange. In particular, it is found that the negative heating gradient in the region of the warm sector of the sur face cyclone is responsible for most of the diabatic exchange across t he tropopause. The mass exchange during the tropopause folding event i s estimated to be around 4.9 x 10(14) kg in four days over the domain considered (1600 X 2000 km). This number is shown to correspond to the net difference between exchange from stratosphere to troposphere (23. 5 X 10(14) kg) and exchange from troposphere to stratosphere (18.6 X 1 0(14) kg). Using the results from the exchange of a passive tracer, th e exchange of ozone is estimated to be of the order of 1.1 X 10(8) kg. Finally, the origin of the air exchanged is found to be from the lowe r stratosphere and the upper troposphere, for the period of four days studied.